A phase shifting transformer (PST) is previously known for controlling the power flow in an ac transmission line. Such PST comprises a tap changer that serially connects or disconnects additional windings of the transformer. By doing so the phasor orientation is controlled. Power is then moved from adjacent phases to a single phase by connections between windings excited by different parts of the magnetic circuit. In a pure phase shifting transformer a voltage in quadrature to the source voltage is injected to the line.
A phase shifting transformer may be used to control the distribution load between parallel lines to increase total power transfer. Advantageous is the capability of the phase shifting transformers to block parasitic power flow due to phase angle difference in a feeding network. Power may be distributed to customer in a defined way and circulating power flows may be avoided.
The use of a PST is advantageous in that the PST has relatively low reactive power consumption. There is no risk of a subsynchronous resonance (SSR) and it is powerful also at low current conditions.
The use of a PST however offers a slow control speed. The tap changer has to go through every tap position in a sequential manner. Each tap change is effected in the order of 3-5 seconds. Thus the PST cannot participate in a decisive way in a transient period following a power disturbance. Further frequent tap changing, in particular at high current conditions, increases the need for maintenance.
The tap changer is a mechanical device and thus slow and an object to mechanical wear. It has a maximum regulation voltage range of 150 kV and the maximum number of operating steps is less than 35. The maximum tap voltage is in the order of 4000-5000V between two tap positions and the maximum rated throughput current is about 3000-4500 A. The maximum power handling capacity is 6000-8000 kVA/tap and there is a short circuit thermal limit. Small voltage steps results in a greater number of mechanical operations.
Another way to control the power flow in an ac transmission line is the use of a controlled series compensator (CSC). Such CSC comprises one or a plurality of thyristor switched inductive devices. The CSC may also comprise one or a plurality of thyristor switched capacitive devices, often in combination with an inductor. The capacitive device or the inductive device is connected in a parallel branch with a thyristor switch. By controlling the thyristor switch the inductive or the capacitive device is connected or disconnected to the transmission line.
Thus the phasor orientation is controlled by connecting or disconnecting a desired number or combination of inductances or capacitances. The regulation is rapid since there is no mechanical switching device involved.
A CSC is controllable from full inductive to full capacitive regulation, and vice versa, within a few fundamental frequency cycles and is thus capable of being a powerful control device in a transient period following a power disturbance. In comparison with the mechanical tap changer of a PST, the need for maintenance of the thyristor controlled CSC does not increase as a consequence of frequent control actions. A CSC is therefore suitable for closed loop control.
However in a circuit comprising a CSC with capacitive steps there is a risk for resonance problems such as SSR. The CSC has a larger reactive power consumption with large inductive steps in comparison to a PST. At low current conditions the CSC has a small impact on the power flow.